Field of the Invention
The present invention relates to a rice male sterile line having a favorable characteristic, a method for producing a rice F1 seed using the same rice male sterile line, and a rice F1 seed which is obtained by the same method.
Description of Related Art
In recent years, remarkable advancements in genome analysis techniques have enabled great improvements to be made to crop production. In particular, a DNA marker technique has showed marked progress and the construction of a new cultivar having a beneficial characteristic has become possible through such a technique. For example, up to now, tomatoes having a resistance to Botrytis cinerea (for example, see Patent Document 1) or rice plants (Oryza sativa) having improvements in lodging resistance and brown rice kernel size (for example, see Patent Document 2) have been created using DNA markers.
Further, through the use of DNA markers and the substitution of chromosome regions including valuable alleles of important genes identified hitherto, specific improvement of a desired characteristic has become possible without significant effect on a large number of other characteristics (for example, see Patent Document 3). For example, as for a rice plant, a rice plant having improvements in culm length (chromosome region in the proximity of sd1 gene), days to heading (chromosome region in the proximity of hd1 gene), number of grains per spike (chromosome region in the proximity of Gn1 gene) or the like has been created (for example, see Patent Document 4). When the sd1 gene in a chromosome of Oryza sativa L. cultivar Koshihikari is substituted with the sd1 gene derived from Habataki, a culm length becomes significantly shorter than Oryza sativa L. cultivar Koshihikari and a lodging resistance is improved. Further, when the Gn1 gene in a chromosome of Oryza sativa L. cultivar Koshihikari is substituted with the Gn1 gene derived from Habataki, a grain density becomes higher than Oryza sativa L. cultivar Koshihikari. When the hd1 gene in a chromosome of Oryza sativa L. cultivar Koshihikari is substituted with the hd1 gene derived from Habataki, this results in conversion to earlier growth than in Oryza sativa L. cultivar Koshihikari.
As for a method of creating a crop having a superior characteristic, there is an F1 hybrid breeding method in which a seed parent is deprived of an ability to synthesize pollen by using a male sterile cytoplasm or the like, whereby crossing between distantly-related lines is realized and the resulting hybrid seed is used as a cultivar. For example, with regard to Lactuca sativa, a Lactuca sativa male sterile line that can be used as a seed parent in an F1 hybrid breeding method has been created (for example, see Patent Document 5).
The F1 hybrid breeding method is used as a technique which is capable of improving yield performance to a very high level with ease by taking advantage of heterosis. Also in breeding of rice plants in Japan, application of the F1 hybrid breeding method has been attempted since the discovery of practical cytoplasmic male-sterility in 1970. In this connection, there is a history that the F1 hybrid breeding method has gradually lost its application due to the fact that taste quality of the line of rice plants reared at that time was not sufficiently high, and a need regarding the high-yielding ability of a rice plant during the rice oversupply period since then is lower.
However, increasing a yield potential of crops has recently become important again in terms of increasing production of food, cultivation costs, and efficient utilization of input energy during cultivation, and will become a more important breeding goal from now on. Further, enlarging a plant itself through the enhancement of productive capacity leads to an increase in productivity of crop residues attracting attention as a raw material of bioethanol of the second generation, and through relative reduction of an amount of GHG discharged in the course of growing processes of crops, may also contribute to a solution to energy problems and environmental problems.
Under the present circumstances in which an improvement of a yield potential has become considered important, an F1 hybrid breeding technique has increasingly gained interest. With regard to an F1 hybrid breeding method, there is a need to create F1 hybrids between large numbers of lines for a candidate line to be selected in a combinatorial test, and therefore the selection of a male sterile line serving as a seed parent has become highly important so as to maintain high efficiency of selection.
Oryza sativa L. cultivar Koshihikari, which is the leading variety in Japan, is evaluated highly regarding taste quality, and the line obtained using Koshihikari as a rearing seed parent has a large number of lines with good taste quality. In addition to taste quality, as shown by the fact that it is most widely cultivated in Japan, Koshihikari has adaptability of cultivation over a wide area and exhibits a great number of excellent characteristics such as germination of strong shoots. Further, since Koshihikari has been used as a study subject in a variety of experiments, Koshihikari has an accumulation of scientific knowledge and has an advantage from the viewpoint that it is easy to find leads for improvement. Taken together, it can be said that Koshihikari is one of the most promising lines in rearing of a seed parent of an F1 hybrid.
Further, with regard to taste quality for which there has been difficulty in term of specifically improving such a characteristic until now, improvement of rice quality became possible through lowering of an amylose content and enhancement of rice glutinosity by taking advantage of a semi-waxiness mutant characteristic. Many rice with semi-waxiness exhibit white turbidity of an endosperm thereof and therefore may be easily distinguished from common rice. Among semi-waxiness mutations reported in the past, there are variations in which 7 different du loci are involved, in addition to a variation of the wx gene which is believed to have the most significant influence on an amylose content (for example, see Non Patent Document 1)